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I'm migrating to MADI.
I just bought a used MADI HDSPe interface with no cable. Digging through my nest of cable, I tried toslink and monster fiber cable but it doesn't work. So, I've been doing some research on what cable to buy and learning the differences.

RME MADI requires either 50/125 µm and 62.5 µm

Like most other MADI devices, the RME MADI products use the common SC-Plug (IEC 874-19) for optical connection. The SC connector is compact and ensures a reproducible and constant connection quality. The SC duplex version combines two fibres. An automatic interlock prevents accidental interruption of the connection. The fibre type, explicitly required for optical MADI connections, must be a 50/125 µm or 62.5/125 µm multi-mode fibre (MM fibre).


62.5- versus 50-micron fiber: Which is better?

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In terms of performance, the difference lies in the fibers' bandwidth, or information-carrying capacity, and in the power-coupling efficiency to light-emitting-diode (LED) sources. Bandwidth is actually specified as a bandwidth-distance product with units of MHz-km. The bandwidth needed to support an application depends on the data rate. As the data rate goes up (MHz), the distance that rate can be transmitted (km) goes down. Thus, a higher fiber bandwidth can enable you to transmit at higher data rates or for longer distances. Therefore, 50-micron multimode fiber offers nearly three times more bandwidth (500 MHz-km) than FDDI-grade 62.5-micron fiber (160 MHz-km) at 850 nm. However, the smaller core of 50-micron fiber can cause a reduction in power budget for LED-based applications. A lower power budget reduces the number of connections permitted in a link and can reduce the sup portable distance for power-limited ap plications like 10-Mbit/sec Ethernet (10Base-F) and Token Ring.

So, while fiber bandwidth is a critical factor in determining link length and data rate, it is not the only one. Transmitter and receiver characteristics also play a critical role. Any statements on the distance capabilities of a particular fiber must be made in the context of the full suite of specifications for a given application. For 850-nm Gigabit Ethernet, these bandwidth values support link lengths of 220 meters over 62.5-micron fiber and 550 meters over 50-micron fiber.

Today, the 850-nm operating window is increasingly important, as low-cost 850-nm lasers such as verti cal-cavity surface-emitting lasers (VCSELs) are becoming widely available for network applications. VCSELs offer users the ability to extend data rates at a lower cost than long-wavelength lasers. Since 50/125 multimode fiber has higher bandwidth in the 850-nm window, it can support longer distances using these lower-cost VCSELs.

Thus, 50/125 is more suitable for fiber backbones running Gigabit Ethernet and higher-speed protocols over longer distances. Either fiber provides sufficient bandwidth for cable lengths up to 300 meters. For many users, that includes their building backbones as well as the horizontal cabling portion of their networks. There are several new 62.5-micron fibers that provide 300-meter-and even up to 500-meter-guarantees for Gigabit Ethernet.

Does this mean you need to recable today if you have old 62.5-micron fiber in your backbone? Not necessarily. The fact that 50-micron multimode fiber performs better with VCSELs at higher speeds is not in itself sufficient reason to recable your current infra structure. Users with an installed base of 62.5/125-micron multimode fiber can still migrate to higher-speed protocols by utilizing 1,300-nm lasers, which will give them the same 550-meter link length as 50-micron fiber.


The background
As today’s networks expand, the demand for more bandwidth and greater distances increases. Gigabit Ethernet and the emerging 10 Gigabit Ethernet are becoming the applications of choice for current and future networking needs. Thus, there is a renewed interest in 50-micron fiber optic cable.

First used in 1976, 50-micron cable has not experienced the widespread use in North America that 62.5-micron cable has.

To support campus backbones and horizontal runs over 10-Mbps Ethernet, 62.5 fiber, introduced in 1986, was and still is the predominant fiber optic cable because it offers high bandwidth and long distance.

One reason 50-micron cable did not gain widespread use was because of the light source. Both 62.5 and 50-micron fiber cable can use either LED or laser light sources. But in the 1980s and 1990s, LED light sources were common. Since 50-micron cable has a smaller aperture, the lower power of the LED light source caused a reduction in the power budget compared to 62.5-micron cable—thus, the migration to 62.5-micron cable. At that time, laser light sources were not highly developed and were rarely used with 50-micron cable—mostly in research and technological applications.

Common ground
The cables share many characteristics. Although 50-micron fiber cable features a smaller core, which is the light-carrying portion of the fiber, both 50- and 62.5-micron cable use the same glass cladding diameter of 125 microns. Because they have the same outer diameter, they’re equally strong and are handled in the same way. In addition, both types of cable are included in the TIA/EIA 568-B.3 standards for structured cabling and connectivity.

As with 62.5-micron cable, you can use 50-micron fiber in all types of applications: Ethernet, FDDI, 155-Mbps ATM, Token Ring, Fast Ethernet, and Gigabit Ethernet. It is recommended for all premise applications: backbone, horizontal, and intrabuilding connections, and it should be considered especially for any new construction and installations. IT managers looking at the possibility of 10 Gigabit Ethernet and future scalability will get what they need with 50-micron cable.

Gaining ground
The big difference between 50-micron and 62.5-micron cable is in bandwidth. The smaller 50-micron core provides a higher 850-nm bandwidth, making it ideal for inter/intrabuilding connections. 50-micron cable features three times the bandwidth of standard 62.5-micron cable. At 850-nm, 50-micron cable is rated at 500 MHz/km over 500 meters versus 160 MHz/km for 62.5-micron cable over 220 meters.

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